Many electric motors employs a diver using pulse width modulation (hereinafter, PWM) inverters in recent years. In the case of driving a motor by a PWM inverter, the neutral point potential of the winding is not zero. This causes a potential difference between the inner and outer rings of the bearings (hereinafter, referred to as the shaft voltage).
A shaft voltage contains high-frequency components caused by switching. If the shaft voltage reaches a value that causes insulation breakdown of the oil film existed in the bearings, a small current flows into the bearings, causing electric corrosion in them. If the electric corrosion proceeds, the inner or outer rings of the bearings or the bearing balls cause undulating abrasion, possibly generating abnormal sound. The sound is one main cause of malfunction of the electric motor.
To reduce electric corrosion, the following technique has been proposed. Patent Literature 1, for example, has proposed a technique in which the rotor includes a dielectric layer. This structure reduces the shaft voltage, and hence, reduces occurrence of the electric corrosion.